Add signature workflow models, services, and configurations

Introduced new models (`SignatureDto`, `SignatureCaptureDto`, `EnvelopeReceiverDto`) to support a signature-based workflow. Added services for handling API interactions (`SignatureService`, `AuthService`, `DocumentService`, `EnvelopeReceiverService`, `SignatureCacheService`).

Enhanced configuration with `ApiOptions` and `PdfViewerOptions`. Integrated DevExpress features with custom data connection providers, in-memory report storage, and font loading utilities.

Marked `AnnotationDto` and `AnnotationService` as `[Obsolete]` in favor of newer implementations. Added detailed documentation for coordinate systems, unit conversions, and usage scenarios.

EnvelopeGenerator.WebUI/EnvelopeGenerator.WebUI.Client/Services/FontLoader.cs

@@ -0,0 +1,12 @@
+using DevExpress.Drawing;
+
+namespace EnvelopeGenerator.WebUI.Client.Services {
+    public static class FontLoader {
+        public async static Task LoadFonts(HttpClient httpClient, List<string> fontNames) {
+            foreach(var fontName in fontNames) {
+                var fontBytes = await httpClient.GetByteArrayAsync($"fonts/{fontName}");
+                DXFontRepository.Instance.AddFont(fontBytes);
+            }
+        }
+    }
+}